Mercury in the Evening

A shy visitor just peeks into view in the west as darkness falls this week. It sets about an hour and a half after the Sun does, so there’s not much time to see it.

Mercury is the solar system’s innermost planet, so it never strays far from the Sun as seen from Earth. At best, it’s visible for a little while before sunrise or after sunset.

It’s putting in a pretty good appearance right now. It’s low in the west as the Sun sets, and slowly becomes visible as twilight begins to fade. You should be able to find it by about a half-hour after sunset, shining like a bright star about 10 degrees or so above the horizon — roughly the width of your fist held at arm’s length. Buildings or trees will block it from view, though, so you need a clear horizon to spot it.

Not only is Mercury the Sun’s closest planet, it’s also its smallest. And those two facts are related.

The planets took form as mountain-sized chunks of rock, metal, and ice merged to form larger and larger bodies. In the outer solar system, that eventually built up giant planets. But in the inner solar system, the heat of the young Sun vaporized the ices in the planetary building blocks, leaving only the rock and metal. So the innermost planets are all small and solid.

Mercury is the smallest of them all, and also one of the densest. It has a large core of iron and nickel surrounded by a layer of rock — a tiny but heavy world that snuggles close to the Sun.

Script by Damond Benningfield, Copyright 2012

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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